This invention relates to electric motors, and more particularly to an electric motor in which a thrust bearing action is performed by the magnetic attraction force between an annular stator core with toroidal coils and a magnet confronting the stator core.
A radial bearing is to prevent the shift of the center of rotation from a predetermined position when a force is applied to the axis of rotation in a direction perpendicular to the latter. Examples of the radial bearing are a fluid bearing such as a kinetic pneumatic bearing, a ball bearing, and a sliding bearing.
A thrust bearing is employed to prevent the displacement of a rotary shaft in the axial direction. In some electric motors, no thrust bearing is employed; that is, the magnetic attraction force between the magnet and the stator core which are confronted with each other is utilized to prevent the axial displacement of the rotary shaft.
A conventional electric motor of this type is as shown in FIG. 11. The electric motor has a kinetic pneumatic bearing as a radial bearing, and drives the polygon mirror of an optical deflector.
In FIG. 11, reference numeral 1 designates a shaft; 1--1, dynamic pressure generating grooves; 2, a housing; 3, a rotary sleeve; 4, a balance adjusting member engaging groove; 5, a yoke; 6, magnets; 7, a stator core; 8, studs; 9, a base plate; 10; a magnetic flux detecting element; 11, the aforementioned polygon mirror; 12, a flange; and 13, screws.
The rotor of the motor is mounted on the shaft 1 with a gap 15; that is, it comprises: the rotary sleeve 3; the yoke 5 and the magnets 6 mounted fixedly on the rotary sleeve 3 by press-fitting or welding; and the polygon mirror 11 mounted fixedly on the rotary sleeve 3 with the flange 12 and the screws 13.
On the other hand, the stator of the motor comprises: the housing 2; the shaft 1 whose one end portion is secured to the housing 2 for instance by press-fitting; the stator core 7 secured to the housing (the toroidal coils (not shown) being wound on the stator core 7); the base plate 9 supported by the studs 8 secured to the stator core 7; and the magnetic flux detecting element 10 mounted on the base plate 9.
The magnets 6 are permanent magnets. Magnetic attraction forces are induced between the stator core 7 and the magnets 6. The attraction forces form a torque producing source, and perform a thrust bearing action to prevent the magnets and the stator core 7 from shifting in the axial direction.
More specifically, when in FIG. 11 the magnets 6 move upwardly, the attraction forces act to move the magnets downwardly; and when the magnets 6 move downwardly, the attraction forces act to move the magnets upwardly. Hence, the magnets 6 and the stator core 7 are held at a predetermined position in the axial direction by the attraction forces so that they confront each other at all times. That is, it can be said that the magnets 6 and the stator core 7 form a magnetic thrust bearing.
The magnetic flux detecting element 10 is made up of a Hall element, for instance. The magnetic flux detecting element 10 is adapted to detect the leakage flux of the magnets 6 thereby to determine whether the N pole has passed or whether the S pole has passed during rotation of the magnets 6.
The detection signal of the magnetic detecting element is applied to a control section (not shown) through a circuit printed on the base plate 9. In response to the detection signal, the control section determines the direction of current flowing in the toroidal coils wound on the stator core 7, so that the mutual action with the magnets 6 produces the magnetic field the polarity of which is such that the rotation is continued.
When the rotary sleeve 3 is rotated, the dynamic pressure generating grooves 1--1 form a high-pressure air layer around the shaft 1 (in the gap 15). This pressure supports the rotary sleeve 3 in such a manner that the latter floats from the shaft 1. That is, a kinetic pneumatic bearing is formed there.
The dynamic pressure generating grooves may be formed in the inner cylindrical wall of the rotary sleeve 3.
The above-described high-pressure air layer acts to maintain the center of rotation of the rotor unchanged. For instance when the rotary sleeve 3 is shifted to the right in FIG. 11, the gap on the right side becomes larger than on the left side, and accordingly the pressure in the gap on the right side becomes smaller; whereas the gap on the left side becomes smaller than on the right side, and accordingly the pressure in the gap on the left side becomes larger. As a result, the rotary sleeve 3 is pushed to the left, and finally it is returned to the original position.
The polygon mirror 11 is polygonal as viewed from above, and has a number of mirrors forming its peripheral sides, to which a light beam such as a laser beam is applied. As the polygon mirror 11 rotates with light beam applied to its first mirror, the light beam reflected therefrom is gradually changed in direction; that is, it is deflected. As the polygon mirror is further rotated, the light beam is applied to the second mirror. Now, the light beam is deflected by the second mirror. Thus, the light beam reflected scans a predetermined angular range. In this case, the scanning speed depends on the speed of rotation of the polygon mirror.
However, the above-described motor suffers from the following difficulties:
(1) The motor is heavy. PA0 (2) The motor is high in manufacturing cost. PA0 (3) There has been a strong demand for miniaturization of end items (such as for instance a light deflector) in which the motor should be built, and accordingly the motor itself should be miniaturized. However, the motor has not been sufficiently miniaturized yet. PA0 (4) Sometimes, dust enters the gap 15 from below (or from the side where the shaft 1 is coupled to the housing 2), thus obstructing the smooth rotation of the rotary sleeve 3, or damaging the surface of the shaft 1 and the inner cylindrical wall of the rotary sleeve 3.
First, the weight of the motor will be described. One of the factors which increases the weight of the motor resides in the rotary sleeve 3.
FIG. 12 is a perspective view showing the rotary sleeve 3 of the conventional motor. The rotary sleeve 3, as shown in FIG. 12, comprises a flange 3-1, a shaft inserting hole 3-2, and screw holes 3--3.
An element to be driven by the motor is mounted on the flange 3-1. The element to be driven is the polygon mirror 11 in the above-described motor.
As is seen from FIG. 12, the flange 3-1 is extended radially of the axis of the rotary sleeve 3. Therefore, it is weak when an axial force is applied to the rotary sleeve, and it may be deformed if the axial force is large. This deformation adversely affects the operation of the element to be driven (hereinafter referred to as "a driven element", when applicable). In the case where the polygon mirror 11 is mounted on the flange, the light beam will not be correctly reflected from the polygon mirror 11.
Accordingly, the flange should be large enough in thickness to prevent the deformation. This requirement increases the weight of the motor.
Next, the manufacturing cost of the motor will be described.
Heretofore, the rotary sleeve 3 is made of stainless steel. Originally, machining stainless steel materials is high in cost; however, the machining cost may be low in the case where the configuration is simple.
The conventional rotary sleeve 3 is not simple in configuration; that is, it has the flange 3-1 with several screw holes 3--3 as shown in FIG. 12. Hence, the machining cost is considerably high.
Since the rotary sleeve 3 is heavy, the rotor is also heavy. The magnets 6, which rotate the heavy rotor and function as the magnetic bearing, are made of rare earth to provide great magnetic forces. The magnet of rare earth is ten to fifty times as high in manufacturing cost as the magnet of ferrite.
These are typical factors for increasing the manufacturing cost of the motor.
Now, the miniaturization of the motor will be described.
The use of the rotary sleeve 3 with the thick flange 3-1 is one of the factors which obstructs the miniaturization of the motor.
Another factor is that the length of the wound coil is not effectively utilized. The motor is rotated by the electromagnetic force acting on the coil in which current flows; however, the electromagnetic force is not produced at all the parts of the wound coil. That is, the electromagnetic force is produced at the part confronting a magnet, but not at the part confronting no magnet. As is seen from the above description, if the ratio of the part of the wound coil where the electromagnetic force is produced to the remaining part is increased, then for production of the same torque the total length of the coil can be reduced as much. This will contribute to the miniaturization of the motor. However, in the conventional motor, the ratio of the part of the coil where no electromagnetic force is produced to the remaining part is large. Thus, the conventional motor is still insufficient in miniaturization.
The last factor, the entrance of dust into the gap 15 from below, will be described.
The entrance of dust into the gap 15 from above may be prevented by the provision of a cover relatively low in manufacturing cost in such a manner that it covers the upper surface of the shaft 1 and the gap 15. However, such a cover for preventing the entrance of dust into the gap from below cannot be manufactured at low cost, because there is the base of the shaft 1 below the gap.